FR2958929A1 - METHOD OF TREATING WATER FOR ITS DESALINATION INCLUDING HIGH SPEED FILTRATION, AND CORRESPONDING INSTALLATION. - Google Patents

Abstract

These objectives, as well as others which will appear later, are achieved by means of a water treatment process with a view to its desalination, said method consisting of: a step (i) of coagulation of said water; a step (ii) of flocculation of water from said coagulation step (i); a step (iii) of granular filtration of water from said flocculation step (ii) through at least one granular filter comprising a filtering mass consisting of at least one layer of at least one filter material; a filtration step (iv) at a cutoff threshold of between 10 nanometers and 10 micrometers of water from said granular filtration step (iii); a step (v) for reverse osmosis filtration of the water from said filtration step (iv) at a cutoff threshold of between 10 nanometers and 10 micrometers; a step (vi) of recovering at least partially desalted water from said reverse osmosis filtration step (iv); said granular filtration step (iii) of passing water from said flocculation step (ii) through said granular filter at a rate between 15 and 25 m / h.

Description

Water treatment process for desalination including high speed filtration, and corresponding installation. FIELD OF THE INVENTION The field of the invention is that of the treatment of water with a view to its desalination. More specifically, the invention relates to such a method which comprises the implementation of a reverse osmosis filtration step. 2. Prior Art The desalination of water (seawater, estuarine waters, industrial waters highly loaded with salts, groundwater contaminated with salts, brackish water, etc.) generally includes a filtration step on one or more reverse osmosis membranes. Manufacturers of this type of filtration membranes do not contractually guarantee the life of their membranes unless the water passing through these membranes have a certain level of quality. Thus, in order to prevent too rapid clogging of the membranes, the water supplying these membranes must have a very good quality. The SDI (Silt Density Index), which is a parameter representative of the clogging power of water, is generally used to characterize the quality level of a water to be treated through reverse osmosis membranes. The SDI15 is the SDI value of a water measured by conforming to the ASTM D4189-95 standardized method (June 15, 2007). The measurement of the SDI15 is carried out as follows. Water is filtered at a constant pressure of 2.1 bar through a filter whose cut-off point is equal to 0.45 micrometers. The time TO necessary for the filtration of 500 ml of water as well as the time T15 necessary for the filtration of 500 ml of water after water has been filtered continuously for 15 minutes through the filter are measured. The SDI15 is then calculated according to the following formula: SDI15 = (100/15). (1- (TO / T15)) The SDI15 index varies between 0 and 6.67, it being understood that the greater its value, the greater the power clogging of the water it characterizes is high. For waters with a very high clogging capacity, SDI15 can not be determined using this standard method. In this case, alternative indices can be calculated by measuring the time required for the filtration of 500 ml of water after water has been filtered continuously through the filter either for 15 minutes but for 3, 5 or 10 minutes depending on the nature of the water to be treated. Manufacturers of reverse osmosis membranes generally recommend that water intended to be filtered through reverse osmosis membranes have, according to the method indicated above, a SDI15 whose value is between 3 and 3.5. Reverse osmosis membrane filtration therefore requires the pretreatment of the feedwater to provide the level of quality required by reverse osmosis membrane manufacturers. In order to produce a water whose SDI15 value is as close as possible to the values in this range, several techniques are currently implemented. A first technique consists in filtering a previously coagulated and flocculated water: in a granular filter comprising a filtering material approximately 1.6 meters high consisting of a layer of anthracite and a layer of sand, at a temperature of speed of the order of 7.5 m / h, then - in a filter cartridge of 5 micrometers. The filter cartridges (for example those supplied by Pall, Sartorius, etc.) are used upstream of reverse osmosis membranes or nanofiltration membranes as protection against particles (with dimensions larger than 5 μm). which could create dysfunctions in reverse osmosis membranes. The filter medium integrated in these cartridges 30 comprises an organic membrane (polysulfone, polyethersulfone, polypropylene, etc.). These cartridges also allow elimination of microorganisms that have dimensions greater than the cutting threshold of the cartridge. The implementation of this technique allows the production of a water whose SDI15 is between 3 and 3.5 only if the raw water to be treated has a SDI3 between 15 and 20. The implementation of a This type of treatment therefore in most cases does not allow the production of water whose quality is sufficient to be optimally filtered later through reverse osmosis membranes.

A second technique consists in implementing a separation step, for example by decantation upstream of the granular filtration step. A third technique is to combine the first two by successively implementing a coagulation, a flocculation, a separation for example a settling, a granular filtration, filtration on filter cartridge and reverse osmosis filtration. The implementation of these second and third techniques can make it possible to produce a reverse osmosis filtration unit feed water of satisfactory quality. 3. Disadvantages of the Prior Art The first of the presently implemented techniques for the purpose of producing reverse osmosis filtration membrane feedwater does not achieve a level of quality which is in accordance with manufacturers' recommendations. of membranes of this type only if the SDI3 of the raw water to be treated is between 15 and 20. The filtration on water reverse osmosis membranes not having the required level of quality is accompanied by an increase of rapid loss of charge at the membranes. This is mainly due to bacterial growth, adsorption of organic matter, and accumulation of microorganic and organic substances on the membranes.

This increase in pressure drop generates the need for frequent chemical cleaning of the membranes. Such cleaning is in practice carried out with aggressive reagents which have a negative impact on the life of the membranes. It is therefore necessary to replace the membranes regularly, which represents a significant cost item. The second and third of these techniques can achieve a level of quality that is consistent with the recommendations of membrane manufacturers of this type. However, they have the disadvantage of being relatively complex and therefore expensive to implement. 4. OBJECTIVES OF THE INVENTION The object of the invention is notably to overcome these disadvantages of the prior art. The invention aims to provide, in at least one embodiment of the invention, a water treatment technique for its desalination by reverse osmosis that allows to extend the life of the reverse osmosis membranes implemented for this purpose. The invention also aims, in at least one embodiment of the invention, to produce such a technique which leads to reduce the clogging time of the reverse osmosis membranes and consequently to reduce the frequency of their washing and their replacement. An object of the invention is to implement, in at least one embodiment of the invention, such a technique which allows the production of a water having a SDI15 of between 3 and 3.5 (measured according to the method ASTM D4189-95) prior to filtration through reverse osmosis membranes. The invention also aims, in at least one embodiment of the invention, to provide such a technique which is simple and inexpensive to implement, at least compared to the techniques of the prior art.

Another objective of the invention is to provide, in at least one embodiment of the invention, such a technique which leads to reduce the area occupied by the facilities used for the desalination of water. The invention still has the objective, in at least one embodiment of the invention, of reducing the quantity of reagents necessary for the desalination of water. 5. Objective of the invention These objectives, as well as others which will appear later, are achieved by means of a water treatment process with a view to its desalination, said process consisting of: a step (i) of coagulation of said water; - a step (ii) of flocculation of water from said step (i) of coagulation; - A step (iii) of granular filtration of water from said step (ii) of flocculation through at least one granular filter comprising a filter mass consisting of at least one layer of at least one filter material; a filtration step (iv) at a cut-off threshold comprised between 10 nanometers and 10 microns of the water coming from said granular filtration step (iii); a step (v) of reverse osmosis filtration of the water coming from said filtration step (iv) at a cutoff threshold of between 10 nanometers and 10 micrometers; - a step (vi) of recovering at least partially desalinated water from said step (iv) filtration not reverse osmosis; said granular filtration step (iii) of passing water from said flocculation step (ii) through said granular filter at a rate between 15 and 25 m / h. Thus, the invention is based on an original approach which consists of the combination of coagulation, flocculation, filtration through a high speed granular filter, i.e. between 15 and 25 m / h, and filtration at a cutoff threshold of between 10 nanometers and 10 micrometers of water for the purpose of producing a feedwater of reverse osmosis filtration. The filtration step at a cutoff threshold between 10 nanometers and 10 micrometers may implement one or more filter cartridges whose cutoff threshold may be between 1 and 101um and will preferably be equal to 5 .mu.m. It may alternatively implement an ultrafiltration filtration unit whose cutoff threshold will preferably be between 10 nanometers and 0.1 micrometers and whose driving force will be between 1 and 5 b ar, or a micro filtration unit whose cutoff threshold will preferably be between 0.1 micrometers and 10 micrometers and whose driving force will be between 0.1 and 3 bar. This particular implementation makes it possible for the flocs present in the water to penetrate rapidly by diffusion in depth inside the filtering mass of the granular filter and to fill, at least in part, the void gaps left between them. media grains filtering essentially over the entire height of the filter media. As opposed to the techniques according to the prior art, the technique according to the invention makes it possible to proscribe the implementation of a separation for example by decantation or by flotation upstream of the granular filtration when the SDI3 of the raw water is situated between 15 and 20. Indeed, considering that the technique according to the invention is based on the implementation of a granular filtration at high speed, the flocs present in the water are retained over substantially the entire height of the filter mass. On the contrary, during the implementation of the techniques of the prior art, these flocs penetrate the filter mass essentially on a low height. A layer of flocs is then formed on the surface of the filtering mass. This clogging of the surface of the filtering mass is accompanied by a rapid increase in the pressure drop across the granular filter. This requires frequent washing of the filters and requires increasing the frequency of chemical cleanings reverse osmosis membranes placed downstream. On the contrary, the use of the technique according to the invention prevents: the formation of a layer of flocs on the surface of the filtering mass, and on the other hand allows the flocs to be absorbed over almost the entire height of the filtering mass. The technique according to the invention therefore makes it possible to retain a large part of the SDI titrating microparticles present in the water and to limit the frequency of cleaning and replacement of reverse osmosis membranes. It is also noted that it makes it possible to retain a much larger share of these particles, given that these fill the empty interstices left between the grains constituting the filtering mass, which makes it possible to retain microparticles having a size smaller than the size of these interstices. All of these advantages allow the technique according to the invention to produce a feedwater of reverse osmosis membranes whose SDI15 is between 3 and 3.5, that is to say which is consistent with recommendations from membrane manufacturers. The filtration step at a cutoff threshold of between 10 nanometers and 10 micrometers is carried out between the granular filtration step and the reverse osmosis filtration step. This filtration, which acts as a fuse, makes it possible, for example when the nature of the water to be treated varies greatly, to ensure that the feed water of reverse osmosis membranes placed downstream has a level of quality. enough to prevent their damage. This implementation makes it possible to extend the life of these reverse osmosis membranes which are very expensive. It is recalled that in a water treatment structure in general, the solid liquid separation rate through a filter is equal to the volume of treated water per hour divided by the filter surface. It is thus possible to determine the surface of a filter knowing the volume of water to be treated per hour and the rate of filtration. The total volume of filter material can then be calculated according to the following relationship: Volume of filter material = Filter area x Filter height. In some cases, the height of a filter can be determined by knowing the filtration rate and the contact time required to perform a given chemical reaction (eg adsorption ...) by the following relation: Filter height = velocity of the filter filtration x time. Preferably, said granular filtration step (iii) uses a granular filter whose filtering mass has a decreasing particle size.

This characteristic makes it possible to promote the penetration of the flocs present in the water over almost the entire height of the granular filter. Indeed, the largest size flocs are retained first letting the smaller size flocs gradually penetrate the core of the filter mass. A process according to the invention is moreover preferably constituted by a sieving step preceding said coagulation step (i). In this case, said sieving step is carried out at a cutoff threshold of between 50 and 500 micrometers. This step is carried out in such a way as to retain the algae and / or the microparticles present in the water to be treated so as to prevent the formation of very large flocs which would clog the surface of the granular filter. The present technique also relates to an installation for the implementation of a method of treating a water for its desalination according to the invention, said installation consisting of: a coagulation zone of said water; - A flocculation zone of water from said coagulation zone; a granular filter comprising a filtering mass consisting of at least one layer of at least one filter material; Means for withdrawing said water from said flocculation zone through said granular filter at a speed of between 15 and 25 m / h; filtration means at a cutoff threshold of between 10 nanometers and 10 micrometers; water from said granular filter (15), and - a reverse osmosis filtration unit of water from said granular filter. The filters used are downflow filters or gravity filters.

The means for withdrawing water from the flocculation zone through the granular filter at a speed of between 15 and 25 m / h may be natural means when the water flows by gravity through the filter or mechanical when water is pumped through the filter. The filtration means at a cutoff threshold of between 10 nanometers and 10 micrometers act as a fuse to protect the membranes of the reverse osmosis filtration unit as has been indicated above. These filtration means may comprise a filter cartridge whose cutoff threshold may be between 1 and 101um and will preferably be equal to 51um. They may alternatively comprise an ultrafiltration filtration unit whose cut-off threshold will preferably be between 10 nanometers and 0.1 micrometers and whose driving force will be between 1 and 5 bar, or a microfiltration unit whose cut-off point will be preferably between 0.1 micrometers and 10 micrometers and whose driving force will be between 0.1 and 3 bar.

Preferably, said filtering mass has a total height of between 2.5 and 4 meters. Such a filter mass height is sufficient to allow the production of water whose SDI15 is consistent with the recommendations mentioned above.

According to a particular embodiment, said first granular filter comprises a stack of two layers of a first filter material and a second filter material, said first and second filter material having decreasing particle sizes. The material at the top has a larger particle size and a lower density than the material at the bottom. This configuration is particularly interesting. During the filtration of the water, the flocs it contains gradually accumulate within the layer of granular material located at the top of the filter. They fill the initially empty interstices that exist between the grains.

This material thus makes it possible to store the flocs present in the water, which themselves have trapped the clogging materials, as well as the suspended solids. The highest material acts as a floc tank, a ripening filter, and allows the removal of SDI. This mechanism occurs after a few hours, that is, once the filter has started to accumulate enough flocs.

The second layer of material located lower in the filter plays a role of refining and retains flocs that may have escaped from the first layer. This is the reason why the particle size of the material of the second layer is always smaller than that of the first layer. According to another advantageous characteristic, said first material has a particle size of between 0.8 and 2.5 mm, and in that said second material has a particle size of between 0.5 and 0.9 mm. In this case, the height of said first material advantageously represents between 50 and 80% of the total height of said filtering mass. The implementation of this characteristic allows a large part of the flocs to diffuse inside the first material layer and thus avoid the formation of a cake on this layer. The clogging speed of the granular filter is consequently reduced, which makes it possible to reduce the frequency of backwashing thereof. The fact that these flocs diffuse inside the first layer, that is to say they are trapped in the interstitial spaces left between the grains of the first layer of material, without clogging the filter also allows to retain other particles whose size is smaller than that of these interstices. Thus, this first layer of material makes it possible to retain most of the SDI titrating particles initially contained in the water to be treated while limiting the clogging of the granular filter.

According to a preferred characteristic, said first material is anthracite and said second material consists of grains of sand or garnet. According to another preferred feature, said first material is pumice stone and said second material consists of grains of sand or garnet. According to another particular embodiment, said first filter comprises a stack of three layers of a first, a second and a third filtering material, said first, second and third filtering materials having decreasing particle sizes.

Further refinement can be achieved by employing a third layer of even more dense and thinner material than the material of the second layer. Such refining makes it possible to increase the removal of suspended particles and to reduce the SDI15 value of the treated water from 0.2 to 0.5. In this case, the height of said first, second and third materials respectively represent between 40 and 75%, between 7.5 and 40% and 7.5 and 20% of the total height of said filtering mass. According to a preferred characteristic, the grains of said materials (17, 18, 17 ', 18', 22) of said layers of said granular filter (15) have increasing densities from the upper layer to the lower layer of said filter.

The fact that the grain density of the materials constituting the different layers of the granular filter is greater from the lower layer to the upper layer allows, after the filter is washed, the various layers of the constituent to reform naturally. Indeed, after the agitation of the layers of materials due to backwashing of the filter has ceased, the densest material grains constituting the lower layer are deposited first while the other grains are deposited in order of decreasing density. An installation according to the invention is furthermore optionally constituted by a sieving unit placed upstream of said coagulation zone. 6. List of Figures Other features and advantages of the invention will appear more clearly on reading the following description of preferred embodiments, given as simple illustrative and non-limiting examples, and the appended drawings, among which: - Figure 1 shows a diagram of a water treatment plant according to a first embodiment of the invention; - Figure 2 shows a diagram of a water treatment plant according to a second embodiment of the invention; FIG. 3 is a curve which illustrates the evolution of the SDI15 of a filtered water according to the prior art in a two-layer filter (anthracite-sand) of 1.6 meters high at 7.5 m / h; FIG. 4 is a curve which illustrates the evolution of the SDI15 of a filtered water according to the prior art in a two-layer filter (anthracite-sand) of 1.6 meters high at 9.5 m / h; - Figure 5 is a curve which illustrates the evolution of the SDI15 of a filtered water according to the invention in a two-layer filter (anthracite-sand) from 3 meters high to 15 m / h. 7. Description of an embodiment of the invention 7.1. General Principle of the Invention The general principle of the invention is based on the combination of coagulation, flocculation, filtration through a granular filter at high speed, that is to say between 15 and 25 m / h, and filtration at a cutoff threshold of between 10 nanometers and 10 micrometers of water for the purpose of producing a feedwater of reverse osmosis filtration. In view of the high water filtration rate, a large portion of the SDI15 titrating flocs present in the water is rapidly absorbed into the filter mass essentially over its entire height without observation of fouling of the surface of the water. filter mass. The implementation of the technique according to the invention therefore makes it possible to produce a water whose SDI15 is between 3 and 3.5. This water can then be optimally filtered through reverse osmosis membranes to be desalted. As opposed to the techniques according to the prior art, the technique according to the invention makes it possible to proscribe the implementation of a separation for example by decantation or by flotation upstream of the granular filtration when the SDI3 of the raw water is situated between 15 and 20. 7.2. Example of a first embodiment of a treatment plant according to the invention In relation with FIG. 1, a first embodiment of a water treatment installation according to the invention is presented.

As represented in this FIG. 1, an installation according to this first embodiment comprises a pipe 10 for supplying a water to be treated in a coagulation zone 11 inside which a coagulating agent is injected which this embodiment is ferric chloride (FeCl 3). The coagulation zone 11 is connected by a pipe 12 to a flocculation zone 13 inside which is injected a flocculating agent which in this embodiment is the FLOPAM AN905 synthetic flocculating polymer. In a variant, a natural flocculating polymer may be used. The flocculation zone 13 is connected by a line 14 to a filter 15. The filter 15 is an open granular filter through which the water to be treated previously coagulated and flocculated circulates under the effect of gravity. In a variant, this filter 15 may be a granular filter under pressure through which the water to be treated circulates under pressure by the implementation of withdrawal means such as a pump. In this embodiment, it comprises a filtering mass 16 which consists of a stack of two layers 17, 18 of two granular filter materials. The layers 17 and 18 constitute respectively the upper layer and the lower layer of the filter. The materials constituting the layers 17, 18 having decreasing granulometries and increasing densities from the top layer 17 to the lower layer 18. The first layer 17 is constituted by anthracite whose particle size is between 0.8 and 2, 5 millimeters. The second layer 18 consists of sand whose particle size is between 0.5 and 0.9 millimeters. In this embodiment, the height of each of the layers of material represents about 50% of the total height of the filtering mass 16.

In variants, the height of the first 17 and second 18 layers of material can vary in proportions such that the height of the first layer 17 can reach up to about 80% of the total height of the filtering mass 16. In a variant the anthracite may be replaced by pumice stone having a particle size of between 0.8 and 2.5 millimeters. In all cases, the sand, preferably rolled or crushed, may be replaced by grains of garnet or any other equivalent material. The total height of the filtering mass 16 can vary between 2.5 and 4 meters depending on the operating conditions. The filter 15 is connected, at its outlet, to the inlet of a filter cartridge 24 by means of a pipe 20. The reverse osmosis unit 19 has a treated water outlet 21. This filter cartridge 24 has a cutoff threshold equal to 5 micrometers. The outlet of this filter cartridge 24 is connected to the inlet of a reverse osmosis filtration unit 19 via a pipe 25. 7.3. Example of a Second Embodiment of a Processing Plant According to the Invention FIG. 2 illustrates a second embodiment which differs from the first embodiment which has just been described with regard to the structure of the filter 15.

In this second embodiment, the filter 15 comprises a filtering mass 16 'which consists of the stack of three layers 17', 18 'and 22 of three granular filter materials having decreasing particle sizes. The first layer 17 ', or top layer, consists of a thickness of 1.6 to 3 meters of anthracite whose particle size is between 1.0 and 2.5 mm. The second layer 18 ', or intermediate layer, consists of a thickness of 0.3 to 1 meter of sand whose particle size is between 0.6 and 0.9 mm.

The third layer 22, or lower layer, consists of a thickness of 0.3 to 0.5 meters of garnet or sand whose particle size is between 0.3 and 0.55 mm. The materials constituting the layers 17 ', 18' and 22 having decreasing granulometries and increasing densities from the top layer 17 'to the lower layer 22. The height of the first layer 17' of material represents between about 40 and 75% of the total height of the filtering mass 16 '. The height of the second layer 18 'of material is between about 7.5 and 40% of the total height of the filter mass 16', and the height of the third layer 22 of material is about 7.5 to 20% of the total height of the filter mass 16 '. The total height of the filtering mass 16 'is between 2.5 and 4.5 meters. 7.4. Variants of the first and second embodiments of a treatment plant according to the invention In a variant of the first and second embodiments (not shown), an installation according to the invention will also consist of a sieving unit placed upstream of the coagulation zone 11. This sieving unit will preferably comprise elements making it possible to retain the algae and / or the microparticles present in the water to be treated having a size greater than 500 microns.

In another variant, the granular filter 15 may be composed of a single layer of sand having a particle size of between 0.5 and 1.5 mm. In other variants, the filter cartridge may be replaced by: an ultrafiltration filtration unit whose cut-off threshold will preferably be between 10 nanometers and 0.1 micrometers and whose driving force will be between 1 and 5 bar, or - a microfiltration unit whose cutoff threshold will preferably be between 0.1 micrometers and 10 micrometers and whose driving force will be between 0.1 and 3 bar.

However, the use of a filter cartridge has the advantage of requiring a driving force and therefore a lower energy consumption than required by ultrafiltration or microfiltration. 7.5. Example of a treatment method according to the invention In relation with FIG. 1, a water treatment process is presented with a view to its desalination according to the invention. Such a method consists in conveying raw water via line 10 into the coagulation zone 11 in such a way that it undergoes a coagulation step therein. The coagulated water from the coagulation zone 11 is then introduced via line 12 into the flocculation zone 13 so that it undergoes a flocculation step. The flocculated water from the flocculation zone 13 is then introduced via the pipe 14 into the granular filter 15. The coagulated and flocculated water then passes through the filtering mass 16 at a speed of between 15 and 25 m / h. The flocs 23 present in this water then penetrate rapidly by diffusion in depth inside the filtering mass 16 and fill, at least in part, the void gaps left between the media grains filtering essentially over the entire height. of the filtering mass. In these circumstances, the granular filter matures much more rapidly.

Given that a portion of these flocs fills the empty interstices left between the grains constituting the filter mass, the microparticles having a size smaller than the size of these interstices are also retained in the filter mass.

The combination according to the invention of coagulation, flocculation and granular filtration at high speed also makes it possible to prevent the formation of a layer of flocs on the surface of the filtering mass, which leads to a reduction in the frequency washing the granular filter. The implementation of this granular filtration step leads to the production of an effluent whose SDI15 has a value between 3 and 3.5. This effluent therefore has a level of quality that is in line with the recommendations of manufacturers of reverse osmosis membranes. This effluent can then constitute a feed water of the membranes of the reverse osmosis filtration unit 19.

Nevertheless, it can happen, quite exceptionally, that this effluent has a SDI15 whose value is slightly greater than 3.5. This may for example be related to the fact that the quality of the raw water has deteriorated. For this reason, the effluent is directed towards the inlet of a filter cartridge 24. This filter cartridge 24 acts as a fuse which retains, if appropriate, the particles present in the effluent at the outlet of the granular filter so that the feed water of the reverse osmosis unit 19 presents in all circumstances a SDI15 whose value is between 3 and 3.5. This effluent is then systematically directed to filtration means with a cutoff threshold of between 10 nanometers and 10 micrometers, which in this embodiment comprise the reverse osmosis filtration unit 19 so as to subtract it, at the very least in part, the salts it contains and to produce desalinated water. In variants, the filter cartridge may be replaced by a filtration unit by ultrafiltration or by microfiltration.

In a variant, the coagulated and flocculated water will be filtered through a tri-layer filter according to the second embodiment. In another variant, the raw water will undergo a sieving step before the coagulation step. 7.6. Other advantages The use of a granular filter according to the techniques of the prior art is insufficient to produce water having the required qualities in order to be subsequently filtered through reverse osmosis membranes. It is therefore necessary to multiply the equipment used so as to reach the required level of quality. This leads to an increase in the size of the facilities. On the contrary, the invention makes it possible to produce water of the required quality, in particular by means of a granular filter operating at high speed and upstream of which it is not necessary to use separation means such as, for example, a decanter or flotation means. Its implementation therefore reduces the overall size and especially the floor area occupied by a desalination plant. The technique according to the invention also makes it possible to reduce the costs associated with the desalination of water. On the one hand, the facilities required for desalination according to the invention are less complex, less bulky and therefore less expensive. On the other hand, the technique according to the invention makes it possible to reduce the washing and replacement frequencies of reverse osmosis membranes. Reducing the washing frequency of reverse osmosis membranes also makes it possible to reduce the water losses used for this purpose. The implementation of the invention also contributes to reducing the volumes of reagents used for the desalination of water. Indeed, the progressive maturation of a filter according to the invention, which is characterized by the fact that it accumulates flocs within its filtering mass during filtration, makes it possible to maintain the adsorption kinetics of the clogging materials. (and therefore SDI) even if the dosage of coagulant initially injected is reduced. This gradual reduction in the amount of coagulant injected during a filtration cycle makes it possible to reduce the overall consumption of reagents. 7.7. Tests Tests were carried out to demonstrate the effectiveness of a water treatment process according to the invention. A first series of tests consisted in filtering, according to the prior art, a water in a filtration column containing a 0.8 meter layer of anthracite and a layer of 0.8 meter of sand, at a rate of 7.5 m / h then in a filter cartridge at 5 micrometers. Figure 3, which illustrates the results of these tests, shows that the SDI15 of the water filtered during these tests was generally greater than 3.5.

A second series of tests consisted in filtering, according to the prior art, the waters of a first station (St 1) and a second station (St 2) in a filtration column containing a layer of 1.5 meters of anthracite and a layer of 1.5 meters of sand, at a speed of 9.5 m / h and then in a filter cartridge at 5 micrometers. Figure 4, which illustrates the results of these tests, shows that the SDI15 of the water filtered during these tests was on average between 4 and 5. A third series of tests consisted in filtering, according to The invention is a water in a filter column containing a 1.5 meter layer of anthracite and a 1.5 meter layer of sand, at a speed of 15 m / h and then in a 5 micron filter cartridge. FIG. 5, which illustrates the results of these tests, shows that the SDI of the water filtered during these tests was always less than 3.5.

Claims (13)

REVENDICATIONS1. A method of treating water with a view to its desalination, said method consisting of: - a step (i) of coagulation of said water; - a step (ii) of flocculation of water from said step (i) of coagulation; - A step (iii) of granular filtration of water from said step (ii) of flocculation through at least one granular filter comprising a filter mass consisting of at least one layer of at least one filter material; a filtration step (iv) at a cut-off threshold comprised between 10 nanometers and 10 microns of the water coming from said granular filtration step (iii); a step (v) of reverse osmosis filtration of the water coming from said filtration step (iv) at a cutoff threshold of between 10 nanometers and 10 micrometers; a step (vi) of recovering at least partially desalted water from said reverse osmosis filtration step (v); said granular filtration step (iii) of passing water from said flocculation step (ii) through said granular filter at a rate between 15 and 25 m / h. 2. Method according to claim 1, characterized in that said step (iii) granular filtration uses a granular filter (15) whose filter mass (16) has a decreasing particle size. 3. Method according to claim 1 or 2, characterized in that it further comprises a sieving step preceding said step (i) of coagulation. 4. Method according to claim 3, characterized in that said sieving step is performed at a cutoff threshold of between 50 and 500 micrometers. 5. Method according to any one of claims 1 to 4, characterized in that said step (iv) of filtration at a cutoff threshold of between 10 nanometers and 10 micrometers uses a filter cartridge or an ultrafiltration unit or a microfiltration unit. 6. Installation for the implementation of a water treatment process for desalination according to any one of claims 1 to 5, said installation consisting of: - a coagulation zone (11) of said water; - a flocculation zone (13) of water from said coagulation zone (11); - A granular filter (15) comprising a filter mass (16, 16 ') consisting of at least one layer of at least one filter material; - means for withdrawing said water from said flocculation zone (13) through said granular filter (15) at a speed of between 15 and 25 m / h; filtration means with a cutoff threshold of between 10 nanometers and 10 micrometers of water from said granular filter (15), and a reverse osmosis filtration unit (19) of the water coming from said filter granular (15). 7. Installation according to claim 6, characterized in that said granular filter (15) comprises a stack of two layers (17, 18) of a first filter material and a second filter material, said first and second filter material having decreasing grain size. 8. Installation according to claim 7, characterized in that the height of said first material (17) is between 50 and 80% of the total height of said filter mass (16, 16 '). 9. Installation according to claim 6, characterized in that said granular filter (15) comprises a stack of three layers of a first (17 '), a second (18') and a third (22) material filter, said first, second and third filter materials having decreasing particle sizes. 10. Installation according to claim 9, characterized in that the height of said first (16 '), second (17') and third (22) materials respectively represent between 40 and 75%, between 7.5 and 40% and 7, 5 and 20% of the total height of said filtering mass (16 '). 11. Installation according to any one of claims 6 to 10, characterized in that the grains of said materials (17, 18, 17 ', 18', 22) of said layers of said granular filter (15) have increasing grain size from the layer upper to the lower layer of said filter. 12. Installation according to any one of claims 6 to 11, characterized in that it is further constituted by a sieving unit placed upstream of said coagulation zone (11). 13. Installation according to any one of claims 6 to 12, characterized in that said filtration means at a cutoff threshold of between 10 nanometers and 10 micrometers belong to the group comprising: - the filter cartridges (24); ultrafiltration filtration units; - Microfiltration filtration units.